Cognitive radios systems use spectrum in an opportunistic manner. The cognitive radio system will search for underutilized spectrum channels in order use them in an agile manner. The cognitive radio is obliged to avoid disturbing the other radio systems (primary subscribers or primary users) that are operating on the same spectrum band. The underutilized frequency channels are found by spectrum sensing.
Spectrum sensing is generally a continuous task: the frequency channels that are used in the cognitive radio network have to be constantly monitored in case a primary user appears in the channel. Thus, optimizing the power-efficiency of spectrum sensing is essential when the cognitive radio nodes are mobile, battery operated devices. It is not power-efficient to perform the same spectrum sensing functionality in each spectrum sensing node in the cognitive radio network. One way to reduce the load of a single node is to perform the spectrum sensing collaboratively, such as is described in the above-referenced US patent application. The spectrum band that is monitored for cognitive use is divided into frequency channels. Each node is sensing a different frequency channel at different sensing time instance. The spectrum sensing results are then shared among the collaborating nodes. Thus, not every node has to sense all frequency channels, but instead the task is split in the frequency domain among the nodes in the same cognitive radio network. If there are more nodes than frequency channels, some nodes will perform spectrum sensing in the same frequency channel simultaneously. This is desired since it allows for diversity gains in the face of propagation and fading and helps to avoid the so-called hidden node problem, in which the primary user cannot be detected by using a single terminal due to channel propagation effects such as shadowing or fading.
As described in that referenced patent application, the collaborative spectrum sensing may be realized by a pseudorandom time-frequency code. The spectrum sensing is performed simultaneously at the collaborating nodes at a sensing time instance. The frequency channel that one node is sensing in a certain sensing time instance is generated by a pseudorandom code that is stored in the memory of the node. The node will perform sensing at the different frequency channels in a pseudorandom order which is determined by the code. Using this scheme, one can avoid the hidden node problem since effectively, different nodes sense different frequency channels at different time instances. Also, there is no need for centralized control to coordinate collaborative spectrum sensing. However, in certain user scenarios the power consumption and load of the network of the spectrum sensing policy described in that patent application may be reduced using embodiments of the invention detailed herein.
Also in the spectrum sensing field, a censoring for collaborative spectrum sensing has been detailed in a paper by J. Lunden, V. Koivunen, A. Huttunen, and H. V. Poor, entitled “CENSORING FOR COLLABORATIVE SPECTRUM SENSING IN COGNITIVE RADIOS”, Proc. of the 41st Asilomar Conference on Signals, Systems, and Computers, Pacific Grove, CA, USA, Nov. 4-7, 2007. In this censoring approach, only relevant information is transmitted between the collaborating nodes. The detection result (i.e., the statistic value) is sent to other users when it is larger than a predetermined censoring threshold. If the detection result is less than the censoring threshold, the result is not sent. The censoring threshold is set by a communications rate constraint which determines the probability of sending the detection result under a null hypothesis, i.e., when there is no signal present. The communications rate constraint can be set very low and then the detection result is transmitted to collaborating users practically only when the signal is detected. When no signal is detected the test statistic value is sent with low probability which is defined by the communication rate constraint. There can alternatively be two or more censoring regions. For example, there can be two censoring thresholds so that very low and very high test statistic values are sent.
What is needed in the art is a further improvement upon power conservation in cognitive radios during the spectrum sensing phase of their operation, preferably in a manner that also reduces the network load.